Saied Baghban, PhD

Classification of Polymetallic Vein Systems in Late Devonian Multiphase W-Mo-Bi and Sn-Zn-Cu-In Mineralization at Mount Pleasant: Deciphering Temporal-Spatial Metal Zonation and REY Enrichment

S. Baghban1, D.R. Lentz1, K. Thorne2, N. Rogers3
1Department of Earth Science, University of New Brunswick, Fredericton, New Brunswick, Canada
2New Brunswick Department of Natural Resources and Energy Development, Canada
3Geological Survey of Canada, Ottawa, Ontario, Canada  

As pathways for mineralizing fluids and recording the evolution of the ore-forming systems, veins are critical features in magmatic-hydrothermal ore deposits. The Mount Pleasant deposit, with its successive intrusive phases and diverse mineralization styles, contains a complex veining system that documents the history of fluid evolution and metal saturation. These deposits are associated with three phases of evolved, Late Devonian A-type granites: fine-grained granite I (hosting W-Mo-Bi), porphyritic granite II (hosting Sn-Zn-Cu-In), and medium-grained granite III, remains unlinked to known mineralization. Vein types and their sequence for each mineralization phase are established by examining mineralogy, crosscutting relationships, and transitions from high- to low-temperature, relatively reduced to more reduced conditions, and low- to high-ƒS₂. Rare earth elements and yttrium (REY) in these vein–veinlets are identified using µXRF-EDS mapping, SEM-BSE and EDS imaging, and cathodoluminescence. During the first mineralization episode, ore deposition is mostly within breccia pipes, vein-veinlets, and stockwork zones. This stage is associated with greisen I (fluorite+topaz+quartz+sericite) alteration, whereas lower-grade ore intervals correspond to greisen II (topaz+quartz+chlorite+biotite). From early to late, the vein–veinlet sequence includes quartz stockwork, wolframite-arsenopyrite, wolframite-native bismuth-arsenopyrite-löllingite-quartz, wolframite-molybdenite-bismuthenite-löllingite-quartz, wolframite-molybdenite-quartz-fluorite, löllingite-fluorite, molybdenite-quartz-fluorite and quartz-fluorite. Spatially and temporally, tungsten and molybdenum veins display a distinct zonation: earlier stages are dominated by distal tungsten-rich veins, with later molybdenum-dominant veins developing deeper and more proximally, often beneath tungsten-bearing zones. The second mineralization event occurs within breccia pipes, lode-vein-veinlet systems, tin-bearing greisen zones, massive sulfide-arsenide replacement zones, and miarolitic cavities, all of which are partially superimposed on the first mineralization stage. The vein-veinlet sequence comprises quartz stockwork, wolframite-arsenopyrite, cassiterite-arsenopyrite-chlorite, cassiterite-arsenopyrite-sphalerite-chalcopyrite-fluorite, sphalerite-chalcopyrite-cassiterite-stannite, galena-sphalerite-stannite-fluorite, sphalerite-tetrahedrite-quartz-fluorite, quartz-fluorite, fluorite-kaolinite, and rhodochrosite veins. This mineralization episode also has a distinct temporal-spatial zonation: tin-rich veins tend to form earlier and closer to the causative granite (mostly in an endogranitic setting), followed by later zinc-copper-lead vein-veinlets in distal settings. Minerals enriched in REY occur in late-stage molybdenite–quartz–fluorite and quartz–fluorite veins as monazite, xenotime, and REY-rich whitish-green fluorite, primarily associated with the first mineralization stage. Based on µXRF-EDS mapping, fluorite displays oscillatory zoning, with REY content increasing from core to rim. Under cathodoluminescence, REY-rich bands appear yellowish-orange to white, whereas REY-poor zones range from greenish-blue to reddish-brown. In conclusion, the Mount Pleasant deposit exhibits a clear temporal-spatial vein-veinlet zoning—transitioning from early W- and Sn-rich to later Mo-, Cu-, Zn-dominant—with REY minerals primarily concentrated in late-stage molybdenite–quartz–fluorite and quartz–fluorite veins.